1. Field of the Invention
The present invention relates to a method for fabricating a metal oxide semiconductor (MOS) transistor, and more particularly, to a method for fabricating a strained silicon channel MOS transistor.
2. Description of the Prior Art
A conventional MOS transistor generally includes a semiconductor substrate, such as silicon, a source region, a drain region, a channel positioned between the source region and the drain region, and a gate located above the channel. The gate composed of a gate dielectric layer, a gate conductive layer positioned on the gate dielectric layer, and spacers positioned on the sidewalls of the gate conductive layer. Generally, for a given electric field across the channel of a MOS transistor, the amount of current that flows through the channel is directly proportional to a mobility of the carriers in the channel. Therefore, how to improve the carrier mobility so as to increase the speed performance of MOS transistors has become a major topic for study in the semiconductor field.
One way to increase the mobility of the carriers in the channel of an MOS transistor is to produce a mechanical stress in the channel. A compressive strained channel, such as a silicon germanium (SiGe) channel layer grown on silicon, has significant hole mobility enhancement. A tensile strained channel, such as a thin silicon channel layer grown on silicon germanium, achieves significant electron mobility enhancement. Another prior art method of obtaining a strained channel is to epitaxially grow a SiGe layer adjacent to the spacers within the semiconductor substrate after forming the spacer.
In this type of MOS transistor, a biaxial tensile strain occurs in the epitaxial silicon layer due to the silicon germanium, which has a larger lattice constant than silicon, and, as a result, the band structure alters, and the carrier mobility increases. This enhances the speed performance of the MOS transistor.
The performance of MOS transistors has increased year after year with the diminution of critical dimensions and the advance of very large scale integrated circuits (VLSI); therefore, the demand for the speed performance of the MOS transistor has also greatly increased. However, the compressive or tensile stress obtained according to the conventional method has been hardly achieved the required extent.
Accordingly, the applicants provide a method of fabricating strained silicon channel MOS transistors to improve the shortages from the prior art, and then increase the carrier mobility of MOS transistors.